It has already been disclosed the the preparation of alkoxysilanes by reacting chlorosilanes with alcohols can be carried out both in the gas phase and in the liquid phase. Gas-phase reactions are described, for example, in DE-A No. 3,236,628 (=U.S. Pat. No. 4,506,087), DE-A No. 2,061,189 (=U.S. Pat. No. 3,792,071) and DE-A No. 3,000,782 (=U.S. Pat. No. 4,298,753). Reactions in the liquid phase, for example the reaction of chlorosilanes with alcohols in a stirred vessel, are described by British Patent Specification No. 674,137. Processes are likewise known in which the hydrogen chloride produced is expelled by means of inert gases (DE-B No. 862,895).
All these processes thus differ from one another in how the hydrogen chloride produced during the esterification is removed. It is absolutely necessary to remove the hydrogen chloride from the reaction mixture quickly in order to ensure complete reaction, to obtain a product which is free of hydrogen chloride, and to prevent undesired side reactions, such as, for example, the formation of polysiloxanes. In order to achieve this, expensive and complicated plants are necessary which, because of the hydrogen chloride produced, must in addition be manufactured from expansive materials. In addition, the removal of the final traces of --Si--Cl groups is difficult.
The method described by A. Gancberg and J. Vandevelde in Industrie Chemique Belge No. 6,591 (1964) for obtaining alkoxysilanes by converting alkyl orthoformates can hardly be achieved industrially if only for cost reasons. In addition, large amounts of alkyl formates are produced during this reaction.